When I purchased them, I found the sub-bass empty and lacking. I heard about seal problems, and clamped them very tight. It made a small difference and I can see how if a pair was looser than mine, there could be an issue.
How variable is the clamping from pair-to-pair out of the box? I don't know the answer to that and I am skeptical. Seems more likely people get a small improvement from clamping (They are very big and cushiony), and then continue to adjust their ear to the very natural sound signature.
The seal issue seems pretty controversial and difficult to test/prove
What we do know is these headphones are tuned for a more natural sounding bass. So it's important to mention that to someone who is planning to spend 300 dollars and is concerned about the seal.
That means you had a seal from the very beginning if it made little to no difference. Your sub-bass issue is with the sound signature itself, then. I also found my first K550 (probably the first batch since I bought it as soon as they were available) lacking sub-bass out of the box. It slightly improved through burn-in but I never found them to have very good extension. However, I purchased a refurbished pair just last week and that had noticeably better sub-bass. I reckon AKG has been tinkering with the K550 since it came out.
IMO the seal issue was never controversial. Seal problems (if they exist) are virtually the same across every closed headphone- you lose much of your sub-bass and the midrange tilts to the right- which becomes pretty unbearable for me.